The analysis of trace compounds present in samples of air or other gases has many applications, for example in studies of atmospheric pollution and in the analysis of breath, which has application in both medical science and in the food industry. Certain compounds present in breath may serve as markers for a particular disease, and the study of compounds responsible for the flavour and aroma of food released during eating is of interest to the food industry. Particularly in the case of organic trace constituents, the compounds to be analysed are usually present in very small quantities in a large volume of gas. For example, the human nose is very sensitive, and odour thresholds in the sub-ppb level are not uncommon (see Teranishi, et.al., in "Standardised Human Olfactory Thresholds", Ed. Devos, pub. IRL Press, Oxford, 1990). In order to characterise the compounds responsible for aromas and taste it is therefore necessary to use analytical techniques having very high sensitivity and specificity, and mass spectrometry is therefore a preferred technique. Also of interest for medical reasons is the measurement of carbon isotopic ratios in exhaled carbon dioxide, a procedure for which mass spectrometry is obviously essential.
One prior off-line method of admitting samples of breath into an analytical instrument such as a mass spectrometer involves the collection of discrete samples of breath in bags or vessels, the contents of which are subsequently analysed by mass spectroscopy. (See, for example, JP patent application pub. no. 60-250227 and Schoeller and Klein in Biomed. Mass Spectrom., 1979 vol 6 (8) pp 350-355.) This method is most successful for the determination of carbon isotopic ratios in exhaled carbon dioxide. More suitably for the analysis of traces of organic compounds, breath may be passed into absorbent (eg, Tenax) or cryogenic traps which collect the organic compounds but not air. The organic compounds may then be subsequently desorbed from the trap and analysed by, for example, gas chromatography-mass spectrometry. This method is commonly employed for atmospheric air sampling. Linforth and Taylor in Food Chem., 1993 vol 48(2) pp 115-20 describe use of the method for the study of the aroma release from foods. However, the method lacks adequate sensitivity for the detection of many food aromas and, being an off-line method, is difficult to use to study the kinetics of the release of aromas during eating.
A further problem with prior mass spectrometric methods in which at least a portion of a complete breath sample is admitted to the mass spectrometer is the depression of the sensitivity of the spectrometer, especially for spectrometers having conventional electron impact or chemical ionization sources, by the large quantity of water always present in such samples. In view of this, and also because of the very large excess of air, a better prior method of analyzing trace organic compounds in breath is provided by the use of membrane inlet systems. These methods involve the passage of exhaled air over a thin membrane (usually silicone rubber) the other side of which is in communication with the mass spectrometer. In this way air and water is excluded from the spectrometer, but the organic compounds will diffuse through the membrane and enter the mass spectrometer. Soeting and Heidema, in Chem. Senses, 1988 vol 13 (4) pp 607-17, and Haring, et.al, in "Flavour Science and Technology", pub. Wiley, Chichester, 1990 teach the use of such membrane inlet systems for the study of the release of flavour compounds at the nose during eating. Membrane inlet systems have also been used for the analysis of trace organic compounds in atmospheric air.
However, membrane inlet systems also have disadvantages. The membrane may exhibit selectivity, excluding some compounds which have a low affinity for the membrane, and some compounds may exhibit very slow diffusion through the membrane and consequently have extended response times. Membranes are also very thin and consequently fragile and of limited lifetime.
Another mass-spectrometric technique which has been used for the analysis of trace organic compounds in breath is direct-introduction atmospheric pressure ionization mass spectrometry (API). In this technique, ions are formed in a sample gas at high pressure (typically atmospheric pressure) by means of a corona discharge or radiation from a suitable source (eg, .sup.63 Ni) and enter a mass analyzer (which operates at high vacuum) through a very small orifice. In use, a gas to be analyzed is caused to flow through a tube in which a discharge electrode is suspended, and ions formed in the discharge pass through an aperture disposed downstream of the electrode into the mass analyzer. (See, for example, GB patent 1582869). Because the ionization takes place at high pressure, the ions formed in API mass spectrometry are typically cluster ions, often comprising a molecule of a trace organic compound clustered with water molecules. The ionization process is a chemical ionization process which may be represented by the following: EQU H.sub.3 O.sup.+ (H.sub.2 O).sub.n +T.fwdarw.TH.sup.+ (H.sub.2 O).sub.m +(n-m+1)H.sub.2 O
where T represents a trace organic molecule present in the sample. The species H.sub.3 O.sup.+ (H.sub.2 O).sub.n is a protonated water cluster ion formed in the discharge in air in the presence of water. More details of API mass spectrometry are given by French, Thomson, Davidson, Reid, and Buckley in "Mass Spectrometry in Environmental Sciences", Eds. Karasek, Hutzinger and Safe, pub. Plenum Press, 1985, at pp 101-120. These authors quote detection limits in the low ppb-ppt range for various organic compounds present in atmospheric air. Benoit, Davidson, Lovett, et.al, in Anal. Chem. 1983 vol 55 pp 805-7 report the use of such an API system for breath analysis. The inlet system used comprised a capillary tube through which breath is introduced into a flow of an inert carrier gas which then enters the mass spectrometer. In order to control the dilution ratio, the subject is required to maintain a constant pressure differential across the capillary while exhaling. A similar system, using a flowmeter to control the dilution ratio, was earlier reported by Lovett, Reid, Buckley, et.al in Biomed. Mass Spectrom. 1979 vol 6 (3) pp 91-97. These systems are capable of providing an analysis of each exhalation but are inconvenient because the subject has to control his breathing to maintain a constant dilution ratio. Particularly when monitoring trace compounds indicative of a disease, however, the apparatus taught by U.S. Pat. No. 5,042,501 may be used. The inlet system described therein incorporates a mixing chamber which intended to average out individual exhalations and produce a constant signal from the mass spectrometer. This apparatus is clearly unsuitable when a very fast response time is required, for example during the analysis of breath for aroma constituents during eating. Also, in U.S. Pat. No. 5,042,501, the breath itself provides the entire flow of gas to the API spectrometer, making the provision of a mixing chamber essential to maintain the gas flow to the spectrometer while the subject inhales.
An off-line method of API mass spectrometric analysis of breath samples is taught in U.S. Pat. No. 4,735,777, but this is inapplicable to most of the applications to which the present application is directed. Further, none of the prior methods of API analysis of breath are suitable for sampling breath from the nose rather than from the mouth, which is highly desirable in the study of the aroma release from foods.